<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>闭包陷阱</title>
</head>

<body>
    <button id="btn">add</button>
    <script>
        function foo() {
            var el = document.getElementById('btn');
            el.onclick = function () {
                console.log(el.id);
            }
        }
        foo();
    </script>
</body>

</html>